Thoughts of an aspiring astrobiologist

As some of my readers know, I have an unusual background. I began my academic career as an evolutionary biologist (Master’s at the University of Rome; Doctorate at the University of Ferrara, Italy; PhD at the University of Connecticut), switching to philosophy (PhD at the University of Tennessee) later on. A number of people, even recently, have asked me why. Here’s the answer, which I offer not (just) as a self indulgent piece of personal biography, but as a reflection on the academic world and the role of serendipity in life. It may be of interest to some, especially young students who are considering a career in either field.

If you ever wanted to know what other planetary systems might look like beyond our own solar system, then NASA’s Eyes is for you. This nifty little app allows you to visit stars known to host their own exoplanets or are potential candidates. Basic information is provided for each exoplanet including its method of detection, mass, orbital period, and size. You can even display where the habitable zone is located and compare a system to our own.

To give an idea of what this program looks like, here are some screen shots:

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NASA’s Eyes also has a suite of apps for stuff closer to home, too. You can track the real time positions and mission highlights of several space craft including Juno, New Horizons, Cassini, and Dawn. You can even free roam and take a tour around the entire solar system! The NASA’s Eyes page can be found here.

In recent days the science sections of the media have been full of the successful entering of orbit around Jupiter by the NASA probe Juno after its five-year, 2.8 billion kilometre journey from the Earth. Many of the reports also talk about the so-called Galilean moons, Jupiter’s four largest moons (there are currently 67 known moons of Jupiter), and Galileo’s discovery of them with the recently invented telescope in early 1610.

Montage of Jupiter’s four Galilean moons, in a composite image depicting part of Jupiter and their relative sizes (positions are illustrative, not actual). From top to bottom: Io, Europa, Ganymede, Callisto. Source: Wikimedia Commons

Juno was even carrying Lego models of the god Jupiter, the goddess Juno and Galileo holding a telescope.

Today, instead of looking at planets from beyond our Solar System, we’re going to look at something closer to home. In January two Caltech researchers, Dr. Konstantin Batygin and Dr. Michael Brown, submitted a paper to The Astronomical Journal hypothesizing the existence of a ninth planet in our Solar System (sorry, Pluto!). This planet would have a mass about ten times that of Earth’s and orbit as far as 1200 AU from the Sun—24 times that of Pluto’s largest orbital distance!

Actually, no direct detection was made. So, you might ask, how did they do it?

First, a Geometry Brief

Before we continue, it will help to define a few terms. Many objects in our Solar System follow an elliptical path as they orbit the Sun:

The perihelion is the distance of a planet’s closest approach, while the aphelion is the furthest approach. The line connecting these two points is called the major axis, and the semi-major axis is simply half that distance. Ellipses are also characterized by their eccentricity, which is typically denoted as e. The eccentricity describes how much the ellipse deviates from being perfectly circular. It takes on values greater than zero (perfectly circular) to less than one (parabolic).

Strange Orbits

The proposed planet (we’ll call it Planet Nine) was inferred from observing objects whose orbits came no closer than 30 AU from the Sun, which is the average orbital distance of Neptune. These objects are termed Trans-Neptunian objects, or TNOs. So, what was it about these orbits that lead the Caltech researchers to suggest a new planet in the first place?

These orbital oddities were noted in a 2014 paper by Trujillo and Sheppard , which described the discovery of a new minor planet with a perihelion of 80 AU, called 2012 VP. They noticed that TNOs with a semi-major axis of over 150 AU were oriented similarly in space. This is strange, considering we would expect these orbits to be randomized by gravitational interactions after their initial formation early in the history of the Solar System.

Clustering of TNOs. All objects have a perihelion greater than 30 AU. The vertical line indicates objects with a semi-major axis greater than 150 AU. (Trujillo & Sheppard 2014)

To explain this, Trujillo and Sheppard purposed that these objects were kept roughly aligned in space by a three-body interaction called the Lidov-Kozai effect. The details of this effect need not concern us, only that it would in principle keep these objects similarly oriented in space. In this scheme, one of the three bodies would actually be another planet. Simulations of this effect could not pin down the exact details of the perturber, but a super-Earth-mass object was plausible.

Planet Nine

This is where our two scientists from Caltech come in. They argued that the Lidov-Kozai effect could not account for the observed orbits. It would require multiple planets with very specific orbits and would predict the existence of a second population of objects in an opposing orientation, which is not observed. The Caltech scientists decided to investigate a different scenario involving a large unseen planet.

Now, in order to do this, the researchers had to make sure that none of the outer planets in the Solar System (especially Neptune) were having a significant influence on the TNOs over the last 4 billion years. After running several simulations, they determined that only six were unaffected. Using these six objects, they ran numerical simulations to determine what parameters the hypothesize planet would need to cause the observed orbits. These calculations suggested an object ten times the mass of the Earth with an eccentricity of 0.6 and semi-major axis of 700 AU.

Orbits of some of the TNOs and that of Planet Nine (called “Planet X” here). From Science.

The Future of Planet Nine

This paper presents the strongest evidence we have yet of a distant unseen planet in our Solar System, but is it really out there? Well, maybe. It would help to have more than a handful of TNOs for analysis to see if other bodies exhibit similar orbital anomalies. Personally, I will be waiting for a direct detection. The researchers began their search in early March using the 8.2 meter Subaru telescope at the Mauna Kea Observatory on Hawaii. Also, a group of French researchers have narrowed down the search area by using data from the Cassini spacecraft and computer models (Fienga et al. 2016).

So, what implications would this ninth planet have if it is found? Such a discovery would help improve our understanding of the formation of the Solar System. A planet this far out from the Sun may suggest that either the early Solar System covered a large region of space or the planet formed closer to the Sun and was later ejected.

I have been so busy with classes lately that I have had little time to blog. But fear not readers! I will have a new astronomy post out soon…my physics classes permitting. In the meantime, let’s have some fun with mnemonics.

Mnemonics are not only useful memory devices but are also really fun to make up. For example, this traditional mnemonic for taxonomic ranks came up in my paleobiology class recently:

King Phillip Cried Out For Good Soup

The ranks in order from most inclusive to least are: kingdom, phylum, class, order, family, genus, and species. Domain can also be added at the very beginning. But this is BORING. A friend of mine back in high school biology came up with a much better one:

Kids Playing Chess On Freeway Go Splat

Morbid? Yes, but I always remember it! There is also a traditional mnemonic used to remember the spectral classes of stars:

The latest data analysis from the Kepler mission was released earlier today during a NASA teleconference. Among its findings were 11 candidate exoplanets less than twice the diameter of Earth orbiting in their habitable zones and another exoplanet that is one of the closest Earth analogs found to date named Kepler-452b. What makes this so exciting is the fact that it orbits a star (Kepler-452) with similar characteristics to our Sun.

Kepler-452b orbits at a distance of just over 1 AU with a period of 384 days very similar to Earth. Based on modeling, the planet is about five times the mass of Earth plus or minus two Earth masses. It is currently not possible to follow up with radial velocity measurements to get a better fix on its mass. Compared to our Sun, Kepler-452 has a similar mass and temperature with a 10% bigger radius.

The star and planet are estimated to be around 6 billion years old making them 1.5 billion years older than our solar system. At this stage in its evolution, Kepler-452 is becoming brighter and hotter which in turn will increase the surface temperature on Kepler-452b over time. This system could serve as a model for what might happen to Earth as our own Sun ages in a similar manner.

Recent research has cast doubt on the potential habitability of Kepler 452b. Analysis of its mass and radius suggests that it is unlikely to be a rocky planet as initially suggested. See here for more details.